Conventionally, there have been considered and developed an electric double layer capacitors having high power and a long service life as a storage device to be mounted in a hybrid vehicle or a fuel cell vehicle.
The electric double layer capacitor stores energy by adsorbing anions and cations onto a positive electrode and a negative electrode, respectively. The stored energy is represented by CV2/2, and a greater energy can be stored by increasing voltage. However, excessively high voltage can cause a chemical reaction (oxidation-reduction reaction) in the positive and negative electrodes, which in turn deteriorates these electrodes.
Therefore, an upper limit of the voltage needs to be set within a range which does not cause any chemical reaction in the positive and negative electrodes, so that the potential range becomes narrow, thereby making it difficult to improve energy density.
In recent years, in order to improve the energy density of the electric double layer capacitor, there has been proposed a hybrid capacitor capable of storing electricity with an electric double layer and also storing electricity by an oxidation-reduction reaction by using a material capable of reversibly carrying lithium ions.
However, even the hybrid capacitor may fail to sufficiently exhibit the capacitance of the positive electrode because an irreversible capacity generates in the negative electrode during a charging/discharging cycle, thereby narrowing the potential range of the positive electrode. That is, the capacitance of the positive electrode cannot be fully utilized, failing to achieve improvement in energy density.
In order to eliminate the above-mentioned disadvantage, for example, Patent Document 1 discloses an electric double layer capacitor having a positive electrode composed of a polarizable electrode material mostly containing activated carbons and a current collector made of aluminium or stainless steel; a negative electrode composed of a carbonaceous material in which lithium ions are inserted into a carbon material capable of carrying lithium ions by a chemical method or an electrochemical method and a current collector which does not form an alloy with lithium; and a non-aqueous electrolyte solution containing a lithium salt.
Further, Patent Document 2 discloses an organic electrolyte capacitor including a positive electrode, a negative electrode, and an aprotic organic solvent solution of lithium salt as an electrolyte solution, in which the positive electrode active material can reversibly carry lithium ions and anions, the negative electrode active material can reversibly carry lithium ions, capacity per unit weight of the negative electrode active material is higher by a factor of 3 or more than that of the positive electrode active material, the weight of the positive electrode active material is higher than that of the negative electrode active material, and lithium ions are preliminarily carried on the negative electrode.
The electrochemical capacitors described in Patent Document 1 and Patent Document 2 can compensate for the capacitance equivalent to the irreversible capacity, for example, by precharging the negative electrode to thereby insert lithium ions into the negative electrode in advance. Therefore, the capacitance of the positive electrode can be fully utilized, thereby achieving improvement in capacitor energy density.
When the lithium ions thus are inserted with a large amount of electricity, the capacitance can be further exhibited in the positive electrode by adsorption and desorption of the lithium ions, so that the discharge potential of the positive electrode is shifted to the lower potential region, thereby extending the potential range. Therefore, energy density can be further improved.    Patent Document 1: Japanese Patent No. 3689948    Patent Document 2: International Patent Publication No. WO2003/003395